The present invention concerns a composition for use in forming a white porcelain enamel coating on aluminized or galvanized steel, a method of forming such a coating on a section of aluminized or galvanized steel, and a section of aluminized or galvanized steel having a white porcelain enamel coating disposed thereon.
Only a few compositions are known that are specifically intended for use in forming white porcelain enamel coatings on aluminized or galvanized steel. An example of one such prior art composition for use on aluminized steel is disclosed in EP 0 950 644 A1. Additional compositions are needed that can be used to form white porcelain enamel coating on aluminized steel. Preferably, such porcelain enamel coatings would exhibit surface properties that are similar to the surface properties of conventional white porcelain enamel coatings formed on sheet steel.
The present invention is directed to a composition that can be used to form a white porcelain enamel coating on aluminized or galvanized steel that exhibits excellent opacity and other highly desirable surface properties. The composition according to the present invention comprises a solids portion comprising a glass component and a white pigment. The glass component comprises one or more glass frits comprising by weight from about 30% to 50% P2O5, from about 15% to about 30% Al2O3, from about 8% to about 40% X2O where X=Na and/or K, up to about 30% TiO2, up to about 12% B2O3, up to about 10% ZnO, up to about 8% SiO2, up to about 7% La2O3, up to about 5% Li2O, and up to about 15% F. The white pigment comprises TiO2.
The method according to the present invention comprises providing a steel substrate having an aluminized or galvanized surface providing a composition for use in forming a white porcelain enamel coating as previously described above, applying the composition to the aluminized or galvanized surface, and firing the composition to form a white porcelain enamel coating. Preferably, the aluminized or galvanized surface is cleaned using an alkaline solution prior to the application of the composition. A white porcelain enamel coating formed according to the method of the present invention will typically have a thickness of from about 20 to about 250 microns, with a thickness of from about 40 to about 150 microns being desired. Firing is typically accomplished at a temperature below the point at which the metal coating applied to the steel substrate begins to melt, with the range of typical firing temperatures being from about 500xc2x0 C. to about 700xc2x0 C.
The invention also provides a section of aluminized or galvanized steel having a white porcelain enamel coating disposed thereon. The coating is formed by applying a composition according to the invention to an aluminized or galvanized surface of the section according to the method of the invention. A section of aluminized or galvanized steel having a white porcelain enamel coating according to the invention will exhibit an acid resistance of AA or better according to ISO 2722, a class 1 bond adherence rating according to EN 12209, and an opacity of L*xe2x89xa794 according to the CIE L*a*b* uniform color space, where L*=116 (Y/Y0)⅓xe2x88x9216 for Y/Y0 ) 0.008858.
The foregoing and other features of the invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed.
The composition according to the present invention is particularly well-suited for use in forming white porcelain enamel coatings on aluminized or galvanized steel such as is typically used in automotive, appliance, building material, cookware, and other applications where rust resistance is desired. Throughout the instant specification in the appended claims, the term xe2x80x9caluminized steelxe2x80x9d should be understood to mean steel that has been coated with aluminum or an aluminum alloy. Typically, sheet steel is aluminized using a dipping process, which results in an aluminum or aluminum alloy coating having a thickness of from about 10 to about 100 microns. When the composition according to the present invention is applied to an aluminum alloy surface, it is preferable for the silicon content of the aluminum alloy to be less than about 15% by weight.
Also throughout the instant specification in the appended claims, the term xe2x80x9cgalvanized steelxe2x80x9d should be understood to mean steel that has been coated with zinc or a zinc alloy. Typically, sheet steel is galvanized using a dipping process or an annealing process, which results in a zinc or zinc alloy coating having a thickness of from about 10 to about 100 microns. In the literature, such steel is typically referred to as galvanized, galvannealed, or galvalumed (steel coated with Znxe2x80x94Al alloy).
The composition according to the present invention preferably comprises a solids portion comprising a glass component and a white pigment. The glass component preferably comprises one or more glass frits. The glass frits comprising the glass component preferably provide a compositions by weight as follows:
In addition to the foregoing, it will be appreciated that the glass component can further optionally comprise minor amounts of other materials such as, for example, MoO3, SnO2, CeO2, BaO, MgO, SrO, and Bi2O3.
The glass frit or frits that comprise the glass component may be prepared utilizing conventional glass melting techniques. A conventional ceramic refractory, fused silica, or platinum crucible may be used to prepare the glass frit. Typically, selected oxides are smelted using continuous, rotary, electrical, or induction smelters at about 1100 to about 1300xc2x0 C. for about 25 to about 60 minutes. The molten glass formed in the crucible is then converted to glass frit using water-cooled rollers and milling equipment. It will be appreciated that the step of producing the glass frit is not per se critical and any of the various techniques well-known to those skilled in the art can be employed.
As noted above, the solids portion of the composition according to the invention also comprises a white pigment, which is preferably milled with the glass component. TiO2 is the most preferred white pigment, and the solids portion of the composition according to the invention preferably comprises at least about 15% by weight TiO2. It will be appreciated that other pigments and/or colorants can also be included in the composition to obtain an enamel having a desired coloring.
The solids portion of the composition according to the present invention can further comprise other mill additions such as pH buffers and texture enhancers. Suitable mill additions include, for example, KOH, NaOH, sodium metasilicate, La2O3, LiF, WO3, MoO3, ZnS, H3BO3, sodium silicate, and K2CO3.
Preferably, the glass component comprises at least about 65% by weight of the solids portion of the composition, with the white pigment and any optional mill additions accounting for the balance. More preferably, the glass component comprises about 75% by weight of the solids portion of the composition with the white pigment and any optional mill additions comprising the balance. Although milling fineness is not per se critical, the solids portion of the composition is preferably milled to a fineness of less than about 5 cc being retained from a 50 cc sample on a 400 mesh sieve.
The composition according to the invention can further comprise a vehicle. The presently preferred vehicle is water, but other vehicles commonly used with enamel compositions could be used. The amount of vehicle used will depend in large part upon the application technique being employed.
The composition according to the present invention can be applied using any of the conventional wet application processes, which are well-known. Suitable wet application processes include spray, dipping, flow coating, and electrophoretic deposition. The disclosures of U.S. Pat. Nos. 5,002,903, 4,085,021 and 3,841,986 are incorporated herein by reference for their teachings relative to the electrophoretic application of porcelain enamel coating compositions.
The composition according to the invention can also be applied by dry application techniques. In such circumstances, a suitable organopolysiloxane is added to the composition so that it may be applied by conventional dry application means such as a standard corona discharge gun. The disclosures of U.S. Pat. Nos. 4,110,487 and 3,928,668 are hereby incorporated by reference for their teachings relative to the dry or electrostatic application of porcelain enamel coating compositions.
Preferably, the composition is applied so as to provide a coating thickness of from about 20 to about 250 microns, and more preferably from about 40 to about 150 microns, subsequent to firing. The coating thickness is not critical, and can be varied based upon the particular application or need.
The method of forming a white porcelain enamel coating on an aluminized or galvanized surface of a steel substrate according to the present invention comprises providing a steel substrate having an aluminized or galvanized surface. Preferably, the aluminized or galvanized surface has a thickness of from about 10 to about 100 microns and is cleaned using an alkaline solution prior to the application of the enamel composition. The method according to the present invention also comprises providing an enamel composition as previously described above, applying the composition to the aluminized or galvanized surface, and firing the composition to form a white porcelain enamel coating.
Firing of the composition is typically accomplished at a temperature within the range of from about 500xc2x0 C. to about 700xc2x0 C. Preferably, firing is accomplished at a temperature of from about 570xc2x130xc2x0 C. for about 4 to about 6 minutes. Firing temperature and times are not per se critical to the invention, but firing should be accomplished below the point at which the metal coating applied to the steel substrate begins to melt.
The present invention also provides a section of aluminized or galvanized steel having a white porcelain enamel coating disposed thereon. The white porcelain enamel coating is formed by applying a composition according to the invention as previously described above to an aluminized or galvanized surface of a section of aluminized or galvanized steel and firing the composition. A section of aluminized or galvanized steel having a white porcelain enamel coating disposed thereon according to the invention will exhibit an acid resistance of AA or better according to ISO 2722 and a class 1 bond adherence rating according to EN 12209. Such a section of aluminized or galvanized steel will also exhibit an opacity of L*xe2x89xa794 according to the CIE L*a*b* uniform color space, where L*=116 (Y/Y0)⅓xe2x88x9216 for Y/Y0) 0.008856.
The following examples are intended only to illustrate the invention and should not be construed as imposing limitations upon the claims;